In recent years, since the demand for high speed processing and high definition content is high, digital MFPs and digital cameras are required to be able to transmit a great number of digital signals at a high speed while satisfying the waveform quality. In order to transmit such high-volume data at a high speed, it is necessary to increase the number of transmission lines or increase the transmission speed. However, since electronic substrates are miniaturized and are extremely dense, it is difficult to increase the number of transmission lines.
Further, when data is transmitted via a cable, the number of cable cores directly affects the cost. Further, according to the increase in the transmission speed, variation in timing of signals due to skew is increased. Accordingly, keeping the set up/hold time becomes difficult. Thus, serial transmission which can transmit large volumes of data at high speed with a small number of transmission lines is widely used.
According to the serial transmission system, low-speed parallel signals such as data signals, address signals, and control signals are serialized and are differentially output to the transmission line. The transmitted serial signals are deserialized on the receiving side so that parallel signals are obtained. As the serial transmission system, US2009-0240994 discusses two types of transmission methods.
The first transmission method is clock synchronous serial transmission. A reference clock signal is transmitted along with several pieces of serialized data. According to this system, since the reference clock signal is transmitted along with the serialized data, synchronization of the reference clock signal and each data signal is very important. Thus, skew of each signal pair needs to be minimized.
The second method is clock-embedded serial transmission. According to this method, a clock bit is added to a serialized data stream and a clock signal is embedded in the data stream. The clock signal and the data signal are recovered on the receiving side. According to this transmission system, since the clock signal is embedded in the data stream, the synchronization condition will be more flexible compared to the condition of the clock signal for the data signal of the clock synchronous serial transmission.
If a high speed signal is transmitted through a long transmission cable that produces power loss, a portion of the signal component may be radiated, having the cable working as an antenna, and affect operations of other apparatuses. Thus, it is necessary to reduce electromagnetic interference (EMI) of the apparatus.
Especially, regarding differential transmission, in some cases, an in-phase component, which is generated according to the waveform difference at the rising edge and the falling edge of the transmitted differential signal, passes through the system ground when it returns. In such a case, a large loop antenna, which causes problems, is generated. Since rejecting of the in-phase component of the differential signal on the driver side leads to small antenna size, it is important to reject the in-phase component in suppressing EMI in the differential transmission.
EMI suppression of clock embedded serial transmission includes the following features. First, a capacitor is inserted in series with the differential transmission line and AC coupling is performed. If a driver and a receiver are connected by a cable, AC coupling is effective in removing the DC electric potential difference of the ground electric potential between the systems. Further, the AC coupling can reduce the threshold difference when the driver and the receiver are products of different manufacturers.
It is important that DC level is kept constant when the AC coupling is performed. Thus, data which is transmitted is coded according to conversion table or by a mathematical expression. This is to set the logical transition data rate between the high level and the low level to 50%. Accordingly, the serial data to be transmitted at the low level or at the high level is not continued for 3 to 4 bits or more.
Thus, regarding EMI from a serial transmission system including a differential transmission line, the level transition of the differential signal is likely to occur in 1 bit period. If such level transition occurs, since an in-phase component, having an integral multiple of 1 bit period, is generated, greater EMI is observed at a frequency of one bit period.
Further, data spectrum transmitted in a square wave is expressed by a sinc function. It is known that a frequency of an integral multiple of one bit period does not have a spectrum. In other words, strong EMI is generated at a frequency where a spectrum of an opposite-phase component of a differential signal is null. For example, if the transmission data rate is 400 Mbps, 1 bit period will be 400 MHz, and no transmission signal spectrum will be seen at the integral multiple. Accordingly, strong EMI is generated.
As an EMI suppression method for the clock embedded serial transmission system, since the EMI occurs at the frequency where an opposite phase signal spectrum necessary in the transmission is null, for example, a band elimination filter or a notch filter is added to the differential transmission line. Since the band elimination filter and the notch filter are useful in rejecting the frequency band where the opposite phase signal spectrum necessary in the differential transmission does not exist, it can efficiently reject the in-phase component which causes EMI with a minimal effect on the transmission signal spectrum.
Thus, conventionally, a band elimination filter which includes a series circuit of an inductor and a capacitor and is connected in parallel with the transmission line is used. To be more specific, as illustrated in FIG. 6A, a pair of transmission lines 107a and 107b is connected to series circuits 1061 and 1062 including inductors 1021 and 1022 and capacitors 1031 and 1032. According to each of the series circuits 1061, 1062, LC resonant circuits are formed. “Microwave Filters, Impedance-Matching Networks, and Coupling Structures” (G. Matthaei/L. Young E. M. T. Jones ISBN: 0-89006-099-1, P725 to P772))